This invention relates to recovery of carbonaceous materials from underground deposits. More specifically, this invention relates to the control of flame front advance in the underground combustion or retorting of hydrocarbonaceous materials.
Numerous hydrocarbonaceous materials are found in underground deposits; for example, crude oil, coal, shale oil, tar sands, and others. One method of recovering energy or hydrocarbon from such underground deposits is by underground combustion. An oxidizing gas such as air, sometimes in conjunction with diluents such as steam, can be provided to an underground combustion or retorting zone so as to combust a portion of the combustible material contained therein and either free hydrocarbon or thereby form materials which are suitable for energy recovery. For example, oxygen or air, and possibly steam, can be passed into a coal deposit so as to form off-gases having combustible materials such as light hydrocarbons and carbon monoxide. These gases can then be combusted directly for heat, or energy recovered such as through power generation. Underground combustion can be used in the recovery of petroleum crude oil from certain types of deposits. Air or oxygen, and steam, is passed into an underground deposit and combustion initiated so hot combustion gases will aid in the recovery of such crude oil. Similar technique can be used in the recovery of oil from tar sands. One important use of underground combustion is in the recovery of oil from oil shale.
The term "oil shale" refers to sedimentary deposits containing organic materials which can be converted to shale oil. Oil shale can be found in various places throughout the world, especially in the United States in Colorado, Utah, and Wyoming. Some especially important deposits can be found in the Green River formation in the Piceance Basin, Garfield and Rio Blanco Counties, and northwestern Colorado.
Oil shale contains organic material called kerogen which is a solid carbonaceous material from which shale oil can be produced. Commonly oil shale deposits have variable richness or kerogen content, the oil shale generally being stratified in horizontal layers. Upon heating oil shale to a sufficient temperature, kerogen is decomposed and a liquid is formed. Oil shale can be retorted to form hydrocarbon either in situ or by surface retorting. In surface retorting, oil shale is mined from the ground, brought to the surface, and placed in vessels where it is contacted with hot materials, such as hot shale or gases, for heat-transfer. Hot retorting temperatures cause shale oil to be freed from the rock. Spent retorted oil shale which has been depleted in kerogen is removed from the reactor and discarded. Some well known methods of surface retorting are the Tosco, Lurgi, and Paraho processes.
Another method of retorting oil shale is the in situ process. In situ retorting of oil shale generally comprises forming a retort or retorting zone underground, preferably within the oil shale zone. The retorting zone can be formed by mining an access tunnel to or near the retorting zone and then removing a portion of the oil shale deposit by conventional mining techniques. About 2 to about 40 percent, preferably about 15 to about 25 percent, of the oil shale in the retorting volume is removed to provide void space in the retorting area. The oil shale in the retorting volume is then rubblized by well-known mining techniques to provide a retort containing rubblized shale for retorting.
A common method for forming the underground retort is to undercut the deposit to be retorted and remove a portion of the deposit to provide void space. Explosives are then placed in the overlying or surrounding oil shale. These explosives are used to rubblize the shale, preferably forming a volume of rubble having uniform particle size permeated by uniform gas passages. Some of the techniques used for forming the undercut area and the rubblized area are room and pillar mining, sublevel caving, and the like. After the underground retort is formed, the mass of rubblized shale is subjected to retorting. Hot retorting gases are passed through the rubblized shale to effectively form and remove liquid hydrocarbon from the oil shale. This is commonly done by passing a retorting gas such as air or air mixed with steam and/or hydrocarbons through the deposit. Most commonly, air is pumped into one end of the retort and a fire or flame front initiated by use of a burner or the addition of hydrocarbon such as natural gas, propane, and the like. Heating causes shale oil to be formed from kerogen in the oil shale, leaving coke on or in the oil shale. Combustion is maintained by the burning of coke on spent or partially spent oil shale, thereby producing hot off-gases suitable for retorting. This flame front is passed or advanced slowly through the rubblized deposit to effect the retorting. It is generally desirable to maintain the flame front uniformly oriented so that no area of the flame front advances substantially ahead of the remaining areas. Most often it is desirable to maintain a planar flame front approximately perpendicular to the desired direction of flame front advance. Not only is shale oil effectively produced, but also a mixture of off-gases from the retorting is also formed. These gases contain hydrogen, carbon monoxide, ammonia, carbon dioxide, hydrogen sulfide, carbonyl sulfide, oxides of sulfur and nitrogen, and low molecular weight hydrocarbons. Generally a mixture of off-gases, water and shale oil are recovered from the retort. This mixture undergoes preliminary separation, commonly by gravity, to separate the gases from the liquid oil and from the liquid water. Off-gases are generally used to preheat a newly formed retort, recycled to a burning retort to act as a diluent or retorting fluid, or burned for power generation or other process heating requirements.
A number of patents describe methods of in situ retorting of oil shale, such as Karrick. L. C., U.S. Pat. No. 1,913,395; Karrick, S. N., U.S. Pat. No. 1,191,636; Uren, U.S. Pat. No. 2,481,051; Van Poollen, U.S. Pat. No. 3,001,776; Ellington, U.S. Pat. No. 3,586,377; Prats, U.S. Pat. No. 3,434,757; Garrett, U.S. Pat. No. 3,661,423; Ridley, U.S. Pat. No. 3,951,456; and Lewis, U.S. Pat. No. 4,017,119 which are hereby incorporated by reference and made a part hereof.
One problem in the underground combustion and retorting of carbonaceous materials such as shale oil deposits is the difficulty in forming and maintaining a uniformly oriented or even flame front. If a portion of the flame front advances more quickly than other portions, large portions of the rubblized matter will be bypassed and will not be effectively retorted and the overall recovery of energy from the deposit will be diminished. This is partially attributable to the difficulty in forming a perfectly uniform rubblized mass with uniform gas passages, and also uniformly passing gas into and out of the retorting area. If a narrow portion of the flame front advances completely through the retorting area, oxidizing gas which is passed into one end of the retort will eventually break through the flame front at the leading position and pass to the off-gas collection system. This will naturally overload the off-gas collection system with oxidizing gas which has not had an opportunity to partake in the combustion process. Therefore, flame front breakthrough can lead to the termination of retorting of an oil shale retort before all of, or even a substantial portion of, the rubblized mass of oil shale is retorted, thereby lowering energy recovery from a retort. Flame front breakthrough can also be dangerous because it can result in a combustible or explosive gas composition in the product recovery zone.
Uren, U.S. Pat. No. 2,481,051, and Cheng Yul Cha, et al., U.S. Pat. No. 4,076,312, teach the use of gas removal means downstream of an in situ oil shale retort for removing off-gases. The figures in these patents show that the gas is removed on a nonselective basis with no attempt whatsoever to selectively draw gases from various points in the retort. There is no teaching or suggestion in these patents that gas can be removed so as to control the position or disposition of the flame front. McCollum U.S. Ser. No. 925,178 now U.S. Pat. No. 4,199,026; Ginsburgh, et al., U.S. Ser. No. 925,176 now U.S. Pat. No. 4,210,867; and Ginsburgh, et al., U.S. Ser. No. 925,177 now U.S. Pat. No. 4,210,868; all filed July 17, 1978, are directed to methods for locating underground combustion. These patent applications are especially directed towards locating the position and disposition of flame fronts advancing through underground in situ oil shale retorts. Ginsburgh, et al., U.S. Ser. No. 925,181 filed July 17, 1978, is directed to a method of controlling the flame front during the in situ combustion of the subterranean carbonaceous material. This patent application teaches locating the position of the flame front and then controlling such flame front through various methods such as the control of gas flowing into the in situ retort. None of the above patent applications teaches or suggests the controlled removal of gases from a retort so as to affect the disposition of a flame front.
It is an object of this invention to provide a method for controlling the flame front advance and position in an underground combustion zone, especially in an underground in situ oil shale retort.
It is an object of this invention to provide a process for the efficient recovery of energy from underground deposits of hydrocarbon so that higher yields of energy can be recovered from a given deposit.
It is an object of this invention to prevent the overloading of off-gas recovery systems attendant to underground combustion processes and preventing dangerous gas compositions in off-gas recovery systems.
It is further an object of this invention to provide a method and apparatus for locating and detecting flame front advance.